1. Field of the Invention
The present invention relates generally to piezoelectric ceramics, and particularly to lead zirconate titanate (PZT) piezoelectric ceramics suitably used for forming a piezoelectric transformer. The present invention also relates to the piezoelectric transformer made from PZT piezoelectric ceramics.
2. Related Art
A conventional piezoelectric transformer made from piezoelectric ceramics is incombustible, is smaller in size and has higher efficiency in comparison with an electromagnetic transformer using a coil. Therefore, recently, application of the piezoelectric transformer is considered for various equipments including an inverter for turning on a cold-cathode tube used as a back light of a liquid crystal display. PZT piezoelectric ceramics is widely used to form the piezoelectric transformer. Various compositions of PZT piezoelectric ceramics are proposed to obtain higher voltage rising ratio and efficiency of the piezoelectric transformer. JP-A-8-151264 discloses such a composition of PZT piezoelectric ceramics.
On the other hand, various structures of the piezoelectric transformer are also proposed. It has been found that a laminated-type piezoelectric transformer formed by laminating plural plates made of piezoelectric ceramics has relatively high voltage rising ratio. However, the laminated-type piezoelectric transformer has plural inside electrodes formed between each adjacent piezoelectric ceramics plates. After the inside electrodes are formed, the piezoelectric ceramics plates are fired at 1,200.degree. C. or higher to form the piezoelectric transformer. Therefore, heat resistance of the inside electrode needs to be 1,200.degree. C. or higher. Accordingly, a relatively low-priced silver (Ag) electrode having relatively low heat resistance can not be used as the inside electrode. As a result, a relatively high-priced platinum (Pt) electrode may need to be used as the inside electrode, and production cost of the piezoelectric transformer is increased.